In recent years, it has been desired to further reduce the size of an electrical storage device. It is indispensable to reduce the thickness of each element (e.g., positive electrode and negative electrode) of the electrical storage device in order to reduce the size of the electrical storage device. However, when the thickness of each element is reduced, the interval between the positive electrode and the negative electrode decreases, and a fatal defect may occur due to a small foreign substance. Patent Literature 1 to 3 propose a foreign substance detection method and a foreign substance removal method in order to suppress the occurrence of a fatal defect due to a small foreign substance, and improve the yield of the electrical storage device.